This invention relates to a vaporizer for vapor generation for chemical vapor deposition and related applications using separated chambers for vapor formation and for chemical vapor deposition with flow restriction between the chambers to permit fluid separator for optimum processing.
In chemical vapor deposition, source vapors are commonly used as reagents to react with substrate surfaces to form thin films on the substrate. The main advantage of using source vapors is the ease and precision with which vapor flow rate can be controlled. The main limitation is that not all the reagents can be easily stored in vapor form at ambient temperatures. Some reagents such as BST (barium strontium titanate), SBT (strontium bismuth tantalum), can be much more easily prepared in liquid form at normal temperature with the addition of solvent. Methods of chemical vapor deposition (CVD) that vaporize a liquid source to generate a source gas are therefore preferred for CVD deposition of materials such as BST, SBT, and similar reagents.
One common method used to generate vapor for chemical vapor deposition is to bubble a gas through a heated liquid reagent. As the gas bubbles through the liquid, it is saturated with the reagent vapor. The vapor is then carried by the gas flow to a chamber for deposition. The bubbler generally works well with a pure reagent in liquid form, but is unsuitable for vaporizing materials for BST and SBT deposition. The reagent used for BST and SBT film deposition usually must be dissolved in a solvent and then vaporized. When such a liquid solution is vaporized in a bubbler, the solvent will evaporate more quickly because of its higher volatility. This will cause the concentration of the reagent material in the liquid solution to increase with time. The output vapor quality from the bubbler, therefore, will change with time, causing difficulty in controlling the deposition rate, and the thickness of the film produced. Another disadvantage is the thermal decomposition of the reagents in the bubbler due to the direct contact of the reagent liquid with the heated surface of the bubbler. This premature decomposition may cause variations in the composition in the deposited films and poor reproducibility in film stoichiometry between different CVD deposition runs. Other disadvantages include the large size of the bubbler and a very rapid change in vaporization rate with operating temperature. Very precise temperature control of the bubbler, therefore, is required.
In U.S. Pat. No. 5,278,138 to Ott et al. a multicomponent liquid precursor is first atomized to form an aerosol having droplet diameters primarily in the 0.1 to 10 xcexcm in diameter range. The aerosol is then mixed with a suitable oxygen-containing carrier gas and injected into a reactor with a heated zone for vaporization and subsequent chemical vapor deposition to produce superconducting thin films, such as yttrium-barium-copper-oxide. Similarly U.S. Pat. No. 5,271,957 to Wernberg et al. describes the formation of LiNbO3 thin films with special electrooptic, ferroelectric and piezoelectric properties by aerosolizing a liquid precursor chemical and introducing the aerosol into a conventional reactor for vaporization and chemical deposition. In both cases, the reactor used is conventional.
In Ott et al. and other prior art, vaporization and deposition is carried out in a single chamber. The addition of a separate heating zone in the reactor allows the liquid source chemical in aerosol form to be vaporized for subsequent deposition on a substrate in the reactor.
To form the vapor, the liquid precursor chemicals are atomized and then vaporized at a temperature high enough for vaporization but not too high, which will cause thermal decomposition of the reagent liquid. However, some thermal decomposition is unavoidable, particularly in practical systems that may have temperature non-uniformity in its heated surfaces. The decomposition products will usually appear in particulate form and be suspended in the gas and vapor mixture. When the mixture gas carrying these particulate contaminants is introduced into the CVD chamber for chemical vapor deposition, the wafer would be contaminated. The film quality would then be poor and the integrated circuit devices incorporating these thin films will then have poor performance characteristics, or become non-functional and need to be discarded.
In addition, to deposit thin form on the wafer by chemical vapor deposition, the CVD chamber must go through a complete cycle of operation. The chamber must first be purged with an inert gas, such as argon or nitrogen. The wafer must then be introduced. The wafer must then be heated to the desired temperature for thin film formation. The gas and vapor mixture from the vaporizer must then be introduced into the CVD chamber and the chamber pressure adjusted to the optimal value for deposition. The condition of the chamber must be maintained for a specific period of time to from a thin film of the desired thickness. The mixture gas flow from the vaporizer into the CVD chamber must then be stopped. The chamber must be purged with an inert gas before another wafer can be introduced for deposition.
The process described above is what is normally used for conventional CVD involving gaseous precursors. Since the precursor chemicals are in gaseous form at room temperature, the various operational steps can be carried out by proper sequencing of solenoid valves, or flow controllers.
In the case of precursor chemicals, which are in liquid form at room temperature, the chemicals can be atomized to form an aerosol and vaporized. However, when the vaporizer is large, and has a large mass, it is impractical to stop the process of vapor generation by turning off the electrical power to the heated surface, and turning it on again when vapor is needed. It is desirable that the vapor be generated continuously, even when it is not needed, and controlling the gas and vapor flow to the CVD chamber by drawing off a portion of the gas and vapor flow as needed. The control of this flow of gas and vapor cannot be the same as those in conventional CVD reactors, since the gas and vapor mixture is at an elevated temperature and cannot be easily controlled.
The present invention relates to a method and apparatus forming a heated gas vapor mixture and passing it through a conduit to a separate deposition chamber.
One aspect of the invention provides a vaporizer using an aerosol generator that atomizes a liquid into small and larger droplets carried in a gas stream at substantially room temperature. The aerosol in the form of a spray is carried into a heated chamber for vaporization as the gas stream moves across a heated wall of the chamber and is discharged. The aerosol generator breaks the liquid into droplets both large and small, that vaporize. The resultant gas/vapor mixture is the mixture that is introduced into a separate chemical vapor deposition (CVD) chamber. The two chamber approach permits optimizing each chamber for its desired function, because they are sufficiently isolated from each other by the connecting conduit to function independently.
The atomization can be accomplished with reagent liquid at or near normal room temperature so that no thermal degradation of the material will take place during the atomization step. The atomized reagent droplets are mixed with a carrier gas flowing into the atomizer to form an aerosol of suspended reagent droplets. This aerosol is then introduced into a vaporizer having a vaporization chamber where the aerosol (including larger droplets) comes in contact with heated wall surfaces. As heat is transferred from the heated surface to the flowing aerosol stream, the gaseous medium around the suspended reagent droplets becomes heated first. The heated gas in turn heats the suspended aerosol droplets to cause them to vaporize.
Direct physical contact between the liquid reagent and the heated surfaces in the vaporization chamber can thus be greatly reduced or avoided. This leads to greatly reduced thermal decomposition of liquid reagent which is caused by direct contact between the liquid droplets and the heated surface. Larger droplets are vaporized upon contact with the heated walls, but decomposition is minimized. The vaporizer is capable of provided a stable source of vapor, with precisely controllable operating characteristics for chemical vapor deposition of metal, semiconductor or insulating thin films and related applications.
There is little clogging of the vaporizer due to material decomposition by direct contact with a heated surface. Flash vaporizing of the reagent liquid is carried out in a rapid and reproducible manner. The method and apparatus in the preferred form achieves a high vaporization rate, with reduced physical size over existing equipment because the vaporizer can be compact but the interior of the vaporization chamber has a large, effective vaporization surface area. This system also increases the rate of vapor output per unit of carrier gas input, thus increasing the vapor/carrier gas mass ratio. The present invention provides a vaporizer with a rapid time response so that vapor is generated the instant the aerosol is introduced into the heated vaporization chamber. The transfer to the separate CVD chamber can be through an orifice, a capillary tube or other restrictive passageway to obtain turbulent mixing for uniform mixing of the vapor and carrier gas. The capillary tube or orifice can be heated to maintain the vapor temperature and avoid vapor condensation. The flow restriction is sized to permit maintaining a proper pressure differential between the vaporization chamber and the CVD chamber.
Also, in one aspect of the invention the vaporizer is run continuously and any excess gas mixture is withdrawn from the output so the correct flow is provided to the CVD chamber. In addition, a heated porous filter can be provided between the vaporizer and the CVD chamber. The heated porous filter maintains the temperature of the vapor to avoid vapor condensation as the gas/vapor mixture passes through the filter.